1. Field of the Invention
The present invention relates to improvements in tempering glass sheets. As is well known, a glass sheet is tempered by a two-step process in which the glass is first heated to an elevated temperature sufficiently high for tempering, and then it is cooled very rapidly to a temperature below the strain point.
Tempering provides glass sheets with a stress pattern in which the glass sheet develops a thin skin of compression stress surrounding an interior stressed in tension. Such a stress distribution makes the glass sheet much stronger than untempered glass so that tempered glass is less likely to shatter than untempered glass when struck by an object. Furthermore, in the less frequent times when an outside force is sufficiently large to cause tempered glass to fracture, tempered glass breaks up into a large number of smoothly surfaced, relatively small particles, which are far less dangerous than the larger pieces with jagged edges that result from the more easily induced fracture of untempered glass.
Typical prior tempering apparatus include nozzles extending from plenum chambers to direct a plurality of air blasts against the opposite major surfaces of a glass sheet. The prior art provides means to impart movement to the nozzles in unison relative to the glass sheet to avoid directing the air blasts against fixed locations on the glass. Fixed air blasts cool the fixed locations opposite the blasts rapidly while other locations adjacent the fixed locations are not cooled as rapidly. Without such relative movement, patterns of iridescence form on the surface of the tempered glass. These patterns of iridescence are very annoying when reviewed in reflection.
The glass tempering art has developed many techniques for imparting relative motion between the arrays of nozzles that face the opposite surfaces of a glass sheet to avoid iridescent patterns. Some of these involve linear reciprocation of the nozzles. Others involve linear movement of glass sheets past a pair of arrays of fixed opposing nozzles. Others involve applying orbital movement (elliptical or circular) of nozzles relative to a glass sheet supported at a fixed position.
The shape of the glass sheet to be tempered and its manner of support between plenum chambers determines the best technique for providing relative movement between the nozzle arrays extending from the plenum chambers and the glass sheet to be tempered. For example, when vertically supported glass sheets are oriented with a sharp bend extending in a given direction, it is known to reciprocate the nozzle arrays in spaced vertical planes along axes parallel or approximately parallel to the given direction. When a flat glass sheet or a gently curved sheet of shallow curvature is tempered, the prior art usually found it most convenient to impart air from nozzles to which are imparted circular orbital movements that overlap corresponding movement of the adjacent nozzles in spaced planes parallel to the position occupied by the glass sheet. Such simultaneous reciprocating or orbital motion of the nozzles has usually been accompanied by movement of the entire plenum chambers and apparatus which supports the nozzles and associated plenum chambers. Such devices have consumed tremendous amounts of energy because of the mass of apparatus elements moved in unison with the nozzles.
In an attempt to reduce energy consumption, apparatus has been developed incorporating two movable frames, one arranged on either side of a flat glass to be tempered. Each frame carries a set of nozzles intermediate the inner and outer ends of the nozzles by means of an elastic connection. Movement of the frames rather than the entire plenum chamber distorts the elastic connection and causes the nozzles to describe overlapping curved paths so that tempering fluid applied under pressure through the nozzles strikes the major surfaces of the glass sheet in moving paths to avoid iridescence. However, the elastic means providing the elastic connection impairs the durability of the tempering apparatus nozzles.
It is also known in the prior art to provide a series of nozzle boxes which are adjustable in orientation and capable of positional adjustment so as to provide a curved space between opposing sets of nozzle boxes that comprises nozzles extending approximately normal to the glass sheet surface and whose free ends define portions of spaced curved surfaces shaped to conform to the shape of the glass. However, the adjustable nozzle arrangements have been such that the nozzle position and orientation, once fixed, cannot be changed to permit the blasts of tempering fluid to sweep relative to the glass sheets without providing either linear reciprocation or orbital movement of the entire nozzle boxes and their support construction in a flat plane depending upon the nature of the flatness or curvature of glass sheet undergoing processing. Such orbital movement or linear reciprocation of the nozzles in a flat plane caused the individual nozzle blasts to depend on the uniformity of glass sheet shape to limit the cyclical variations in nozzle to glass distance and orientation of the nozzle blasts relative to the glass surfaces, which were considered factors in promoting non-uniform tempering of shaped glass sheets.
Other prior art patents constrain the movement of arrays of nozzles to curved paths that maintain the nozzles in orientations that remain approximately normal to different portions of the curved surfaces of the glass sheet undergoing quenching as the nozzles move relative to the major glass sheet surfaces. Such apparatus moves the nozzle position controlling means as well as the nozzles and requires considerable energy to move the mass of equipment forming such means.
Other prior art patents suggest different techniques to reduce the effect of buffeting, such as providing opposing arrays of tempering fluid blasts that are 180 degrees out of phase against the opposite major glass sheet surfaces, and directing blasts obliquely downward against the opposite glass sheet surfaces. Other patents suggest out of phase movement to reduce iridescence or to develop various small, non-uniform particle sizes.
2. Description of the Prior Art
U.S. Pat. No. 1,999,337 to Meer tempers glass sheets by combining orbital movement of a casing with continuous rotation of two rows of apertured tubes to change both the direction and relative location of blasts of tempering medium on a glass sheet surface. The opposite surfaces are never acted upon at two exactly opposite points to obtain different size fragments ranging from that of a pea to that of a grain of wheat when the tempered glass is fractured.
U.S. Pat. No. 2,271,377 to Monnet provides relative movement between fluid blasts and flat glass surfaces by arranging nozzles in angled pairs and cycling the feed of blasts to each nozzle in each pair 180 degrees out of phase with one another. This apparatus requires continuous operation of valves in the fluid feed lines to which access is difficult.
U.S. Pat. No. 2,724,215 to Gilstrap discloses a conventional glass tempering apparatus provided with tubular air plenums that are reciprocated to sweep over the surface of flat glass sheets supported between sets of opposing nozzles for cooling during a tempering operation. The nozzles move in unison with their plenum chambers in flat, parallel planes.
U.S. Pat. No. 3,024,572 to Richardson and U.S. Pat. No. 3,717,449 to Seymour show individual elongated plenums that are fixed in position and that can be adjusted to provide a curved space corresponding to the curve of the glass between opposite sets of plenums. These patents also provide means to adjust the orientation of individual nozzle boxes so that the apertures in apertured walls can be directed normal to the corresponding portions of curved glass sheets relative to which they move during the cooling step of tempering. However, the entire system is moved in unison while quenching a shaped glass sheet.
U.S. Pat. No. 3,214,256 to H. W. Baker reciprocates opposing sets of nozzles 180 degrees out of phase with one another in spaced flat planes along paths normal to the axes of bending of sharply bent glass sheets to have the opposite sets of nozzles at equal varying distances from the adjacent curved glass sheet surfaces to reduce the tendency of the curved glass sheet to move toward its sharply bent end portions during quenching. The gist of this technique is to have the nozzles that face the opposite surfaces of any given region equally spaced from the surfaces of said given region even though the equal spacing for one region is different from the equal spacing for another region at most parts of the cycle of nozzle movement.
U.S. Pat. No. 3,279,906 to R. N. Baker applies blasts of tempering fluid through obliquely downward directed holes in the shaping walls of complementary shaped press bending molds to reduce buffeting of the glass sheet.
U.S. Pat. No. 3,918,950 to Stilley moves opposing sets of nozzles in spaced flat planes in closed circular orbits that are 180 degrees out of phase with one another and that are misaligned from side to side of the glass sheets undergoing quenching so as to minimize the iridescent patterns that result from uneven cooling of the glass sheets.
U.S. Pat. Nos. 2,790,270 to Freiberg; 2,876,592 to Black and Moorhead and 2,876,593 to Neuhausen show tempering apparatus that control the movement of nozzles in curved paths and maintain the nozzle orientation substantially normal to each localized portion of the curved glass sheet during relative movement. Power is required to move the nozzles and the position control means for the nozzles, which are heavy.
U.S. Pat. No. 3,454,388 to Ritter reciprocates elongated nozzles having elongated slots about the longitudinal axes to play air blasts provided from rigidly supported apertured pipes over the opposite major glass sheet surfaces. No provision is made for making this apparatus suitable for tempering glass sheets other than those that are shaped to a flat configuration.
U.S. Pat. No. 3,476,541 to Ritter shows rotatable tubular members that cooperate with deflectors to play air blasts over glass surfaces to provide uniform cooling. The tubes are apertured, are polygonal in cross section with apertures in adjacent walls of the polygon offset from one another, and rotate about fixed axes of rotation in a single direction and are fixed in position about their axes of rotation.
U.S. Pat. No. 3,598,562 to Angely discloses apparatus for tempering flat glass sheets in which each nozzle of a set is connected to a plenum chamber and extends through an elastic pivotable connection to a plate so that movement of the plate intermediate the plenum chamber and the orifice of the nozzle causes the nozzles to move in unison in a path defined by the movement of the movable plate on each side of the glass sheet. No provision is made for adjusting the curvature defined by the nozzles to conform to a curved glass sheet.